1. Field of the Invention
The present invention relates to an optical pick-up device and more particularly, a holographic optical element in an optical pick-up device, which can record and reproduce data on optical disks of various formats.
2. Description of the Related Art
Recently, with an optical disk becoming a high-density and high capacity one, the configuration of an optical pick-up device has been developed in the direction of increasing the numerical aperture of an objective lens and reducing the wavelength of a laser beam in order to reduce the size of a beam for recording and reproducing it.
However, at this time, since the deterioration characteristic of a signal about the slant of a disk is inversely proportional to the wavelength of a laser, a tilt margin for the slant of a disk is reduced sharply as density is increased. Accordingly, in order to compensate the tilt margin, as the recording density of a disk is increased, the disk slant characteristic can be compensated by reducing the thickness of a disk board. For example, in a CD disk that has the capacity of 650 MB, laser wavelength is 780 nm and the numerical aperture of an objective lens is 0.45. Meanwhile, in a DVD disk, which has the capacity of 4.7 GB, laser wavelength is 560 nm and the numerical aperture is 0.6. The thickness of a disk in a CD is 1.2 mm and the DVD is 0.6 mm. Also, in the case of a HD disk, which has been developed recently, it has been suggested as a specification that wavelength be 405 nm and numerical aperture be 0.85. In this case, the disk thickness will be 0.1 mm that is a lot thinner, so the slant margin of a disk can be secured.
As the disk thickness is different according to a disk type, if other kinds of disks are recorded and reproduced with an optical pick-up device suited to only a certain type of disk, spherical aberration can be generated greatly clue to the thickness difference of a disk and the degradation of optic quality can occur. Thus, it will be difficult to record and reproduce a signal normally. For this reason, several methods have been suggested that can secure compatibility among the disks, which are different in the thickness of a board.
For example, the aberration shape, which is generated by using an optical pick-up device, which adopts an objective lens designed for a disk of 0.1 mm thickness (which is the HD disk) when a DVD disk of 0.6 mm thickness is reproduced, is shown in FIG. 1. The aberration value, which is converted into the rms value, is about 0.507 λ, but it is very short in Marechal Criterion that the aberration of an entire optical system should be below 0.07 λ. Accordingly, in order to compensate this phase, it has been suggested that an element should be made to have a phase value that is opposed to aberration when a beam having 650 nm wavelength has been incident therein.
In these methods, proposed are several methods that use a holographic optical element (HOE), which is an optical element that adjusts a phase by using the diffraction effect of a ray. FIG. 2 is a view illustrating a state of focusing a beam onto the HD disk and a DVD disk by using an optical pick-up device, in which a holographic optical element and an objective lens, which are designed for a related high-density disk, are adopted.
As shown in FIG. 2, the beam made to be focused onto a HD disk 23 and a DVD disk 24 through an objective lens 22 by controlling an optical route on a beam for DVD and a beam for the HD by using a holographic optical element 21. Here, the objective lens 22 is a lens designed to be suited for use in the HD. Also, the holographic optical element 21 is placed in front of the objective lens 22 and the characteristic of a beam is made to be unchanged by using the 0th ray for a beam of 405 nm wavelength, and for a beam of 650 nm wavelength, its spherical aberration can be compensated by using the 1st ray.
FIG. 3 illustrates the diffraction efficiency of the 1st ray for use in DVD and the 0th ray for use in HD according to the depth of a holographic optical device, in the case that a related holographic element is used in an adopted optical pick-up device. With reference to FIG. 3, the greater the diffraction efficiency of the 0th ray in 405 nm wavelength is, the efficiency of the 1st ray in 650 nm wavelength is decreased. According to this, if the efficiency of the 0th ray in 405 nm wavelength is designed to be large, the efficiency of the 1st ray in 650 nm wavelength is decreased.
Also, provided is another method that uses the 1st ray for both disks. In this case, the objective lens should not be designed with a lens for HD only, but it should be designed to be suited to the characteristic of a source ray that is used on both disks. FIG. 4 shows the diffraction efficiency of the 1st ray for DVD and the 1st ray for HD according to the depth of a holographic optical element in this case.
As shown in FIG. 4, since the points of indicating maximum efficiency according to the depth of a holographic optical element, are different, it is evident that it is difficult to achieve maximum efficiency for both kinds of disks. Also, a shortcoming occurs in that, if an appropriate intermediate value is taken, variation will be serious because of the great diffraction efficiency according to the depth variation.
In other words, when the 0th ray and the 1st ray are used, the efficiency of the 0th ray is substantially decreased if a holographic optical device is designed to have the greater efficiency for the 1st ray, and the efficiency of the 1st ray is substantially decreased if a holographic optical device is designed to have the greater efficiency for the 0th ray, so there is no choice but to take appropriate intermediate value.
Also, because the wavelengths of two rays are different in the case that the 1st ray is used for both disks, it is impossible to design both wavelengths to reach maximum efficiency simultaneously. Accordingly, in this case, the design is the same in that the holographic optical device is designed to have an appropriate intermediate value.
Accordingly, in the case of a general non-polarizing Hologram optical device, it is difficult to reach maximum value in diffraction efficiency for both wavelengths, so there is a limit in that some degree of optical loss occurs.